THE EARLY HISTORY OF THE OVUM. 205 



THE EARLY HISTORY OF THE OVUM. 



The formation of the essential parts of the sexual or repro- 

 ductive cells from the germinal epithelium was mentioned upon 

 a preceding page (p. 125). A brief account of their subsequent 

 history follows. Space does not admit of any extended account 

 of the details of the phenomena of reduction division, matura- 

 tion, impregnation, etc., and the theories based upon them ; for 

 these, reference must be made to the special text-books upon 

 cytology and embryology. 



Both eggs and spermatozoa, as they leave the gonads, are 

 cells specialized for the perpetuation of the species ; and ulti- 

 mate analysis shows that as they leave the parent tissue these 

 cells contain all the absolute essentials for the reproduction of 

 the kind. In the vertebrates, however, as in most other animals, 

 these essentials are variously modified in shape and in composi- 

 tion by the addition of certain secondary features which demand 

 attention. 



As has already been said, the ovum is a specialized cell, 

 which passes into an ovarian follicle, and receives nourishment 

 from the foUicular cells, and grows larger than the other cells of 

 the body. At last it escapes from the ovary, passes into the 

 ccelom, and thence to the exterior either through the Miillerian 

 ducts (most vertebrates), through the pori abdominales (some 

 teleosts), or by means of special structures (many teleosts). In 

 its simplest condition an ovum is directly comparable to any 

 other cell of the body, consisting merely of a mass of proto- 

 plasm with a specialized portion, the nucleus, near the centre. 

 In most cases, however, it receives other parts of a secondary 

 character, either from the ovarian tissues, or from the walls of 

 the ducts through which it passes. 



From the ovarian tissue the egg receives a varying amount 

 of food yolk or deutoplasm. This is a peculiar substance to be 



